What it does:
Stealth Rock can be considered a staple for many teams and both helps and hinders combatants depending on their side of the field. Being able to postpone or altogether prevent the rocks from being set up on your field can save you a lot of grief.
Why it is good:
Sawk is already recognized as a top threat in NU, but with its newly released Dream World ability and the reintroduction of hail, Sawk has gotten even better. Now not only can he break the Sturdy leads that could set up so easily before, the Ice types that came along with Hail gave him even more targets to destroy.
Most Sawk run Choice Band nowadays to brutalize every slower non-fighting resist, but the fact that Jolly Life Orb Sawk can OHKO Piloswine is already a testament to his power. Life Orb Sawk can also run Taunt to stop the likes of Weezing, Armaldo, Garbodor, Torkoal, Alomomola, Regirock and other defensively inclined opponents from doing shit. Choice Scarf Sawk can outrun stuff, but it is getting used less simply because Choice Band or Life Orb Sawk comes out on top of most leads, hazard setters and opponents in general. Scarf Sawk does trade power for speed though, but not killing things like 90% Samurott or 75% Serperior can be a turn-off.
What counters it:
Bulky Psychic types, they are public enemy number one. Musharna, Duosion, Thick Fat Grumpig, you name it. No, not freakin' Chimecho. While Musharna is stunted by Hail (another reason Sawk became better), Duosion usage is increasing steadily. Faster Pokemon like Kadabra, Swellow and Electrode also revenge it, especially after Close Combat.